Enzyme levels in individual rat muscle fibers

Electrophysiology Methods for Assessing of Neurodegenerative and Post-Traumatic Processes as Applied to Translational Research

Electrophysiological studies have long established themselves as reliable methods for assessing the functional state of the brain and spinal cord, the degree of neurodegeneration, and evaluating the effectiveness of therapy. In addition, they can be used to diagnose, predict functional outcomes, and test the effectiveness of therapeutic and rehabilitation programs not only in clinical settings, but also at the preclinical level. Considering the urgent need to develop potential stimulators of neuroregeneration, it seems relevant to obtain objective data when modeling neurological diseases in animals. Thus, in the context of the application of electrophysiological methods, not only the comparison of the basic characteristics of bioelectrical activity of the brain and spinal cord in humans and animals, but also their changes against the background of neurodegenerative and post-traumatic processes are of particular importance. In light of the above, this review will contribute to a better understanding of the results of electrophysiological assessment in neurodegenerative and post-traumatic processes as well as the possibility of translating these methods from model animals to humans.

Characteristics of Muscle Fiber-Type Distribution in Moles

Moles are a strictly fossorial Soricomorpha species and possess a suite of specialized adaptations to subterranean life. However, the contractile function of skeletal muscles in moles remains unclear. We compared muscle fiber-type distribution in two mole species (the large Japanese mole and lesser Japanese mole) with that in four other Soricomorpha species that are semi-fossorial, terrestrial, or semi-aquatic (the Japanese shrew-mole, house shrew, Japanese white-toothed shrew, and Japanese water shrew). For a single species, the fiber-type distribution in up to 38 muscles was assessed using immunohistochemical staining and/or gel electrophoresis. We found that slow and fatigue-resistant Type I fibers were absent in almost all muscles of all species studied. Although, the two methods of determining the fiber type did not give identical results, they both revealed that fast Type IIb fibers were absent in mole muscles. The fiber-type distribution was similar among different anatomical regions in the moles. This study demonstrated that the skeletal muscles of moles have a homogenous fiber-type distribution compared with that in Soricomorpha species that are not strictly fossorial. Mole muscles are composed of Type IIa fibers alone or a combination of Type IIa and relatively fast Type IIx fibers. The homogenous fiber-type distribution in mole muscles may be an adaptation to structurally simple subterranean environments, where there is no need to support body weight with the limbs, or to move at high speeds to pursue prey or to escape from predators. Anat Rec, 302:1010-1023, 2019. © 2018 Wiley Periodicals, Inc.

Dietary enrichment with fish oil prevents high fat-induced metabolic dysfunction in skeletal muscle in mice

High saturated fat (HF-S) diets increase intramyocellular lipid, an effect ameliorated by omega-3 fatty acids in vitro and in vivo, though little is known about sex- and muscle fiber type-specific effects. We compared effects of standard chow, HF-S, and 7.5% HF-S replaced with fish oil (HF-FO) diets on the metabolic profile and lipid metabolism gene and protein content in red (soleus) and white (extensor digitorum longus) muscles of male and female C57BL/6 mice (n = 9-12/group). Weight gain was similar in HF-S- and HF-FO-fed groups. HF-S feeding increased mesenteric fat mass and lipid marker, Oil Red O, in red and mixed muscle; HF-FO increased interscapular brown fat mass. Compared to chow, HF-S and HF-FO increased expression of genes regulating triacylglycerol synthesis and fatty acid transport, HF-S suppressed genes and proteins regulating fatty acid oxidation, whereas HF-FO increased oxidative genes, proteins and enzymes and lipolytic gene content, whilst suppressing lipogenic genes. In comparison to HF-S, HF-FO further increased fat transporters, markers of fatty acid oxidation and mitochondrial content, and reduced lipogenic genes. No diet-by-sex interactions were observed. Neither diet influenced fiber type composition. However, some interactions between muscle type and diet were observed. HF-S induced changes in triacylglycerol synthesis and lipogenic genes in red, but not white, muscle, and mitochondrial biogenesis and oxidative genes were suppressed by HF-S and increased by HF-FO in red muscle only. In conclusion, HF-S feeding promotes lipid storage in red muscle, an effect abrogated by the fish oil, which increases mediators of lipolysis, oxidation and thermogenesis while inhibiting lipogenic genes. Greater storage and synthesis, and lower oxidative genes in red, but not white, muscle likely contribute to lipid accretion encountered in red muscle. Despite several gender-dimorphic genes, both sexes exhibited a similar HF-S-induced metabolic and gene expression profile; likewise fish oil was similarly protective in both sexes.

A comparison of exercise type and intensity on the noninvasive assessment of skeletal muscle mitochondrial function using near-infrared spectroscopy

Near-infrared spectroscopy (NIRS) can be used to measure muscle oxygen consumption (mVO(2)) using arterial occlusions. The recovery rate of mVO(2) after exercise can provide an index of skeletal muscle mitochondrial function. The purpose of this study was to test the influence of exercise modality and intensity on NIRS measurements of mitochondrial function. Three experiments were performed. Thirty subjects (age: 18-27 yr) were tested. NIRS signals were corrected for blood volume changes. The recovery of mVO(2) after exercise was fit to a monoexponential curve, and a rate constant was calculated (directly related to mitochondrial function). No differences were found in NIRS rate constants for VOL and ES exercises (2.04 ± 0.57 vs. 2.01 ± 0.59 min(-1) for VOL and ES, respectively; P = 0.317). NIRS rate constants were independent of the contraction frequency for both VOL and ES (VOL: P = 0.166 and ES: P = 0.780). ES current intensity resulted in significant changes to the normalized time-tension integral (54 ± 11, 82 ± 7, and 100 ± 0% for low, medium, and high currents, respectively; P < 0.001) but did not influence NIRS rate constants (2.02 ± 0.54, 1.95 ± 0.44, 2.02 ± 0.46 min(-1) for low, medium, and high currents, respectively; P = 0.771). In summary, NIRS measurements of skeletal muscle mitochondrial function can be compared between VOL and ES exercises and were independent of the intensity of exercise. NIRS represents an important new technique that is practical for testing in research and clinical settings.

EMG-normalised kinase activation during exercise is higher in human gastrocnemius compared to soleus muscle

In mice, certain proteins show a highly confined expression in specific muscle groups. Also, resting and exercise/contraction-induced phosphorylation responses are higher in rat skeletal muscle with low mitochondrial content compared to muscles with high mitochondrial content, possibly related to differential reactive oxygen species (ROS)-scavenging ability or resting glycogen content. To evaluate these parameters in humans, biopsies from soleus, gastrocnemius and vastus lateralis muscles were taken before and after a 45 min inclined (15%) walking exercise bout at 69% VO2_max aimed at simultaneously activating soleus and gastrocnemius in a comparable dynamic work-pattern. Hexokinase II and GLUT4 were 46–59% and 26–38% higher (p<0.05) in soleus compared to the two other muscles. The type I muscle fiber percentage was highest in soleus and lowest in vastus lateralis. No differences were found in protein expression of signalling proteins (AMPK subunits, eEF2, ERK1/2, TBC1D1 and 4), mitochondrial markers (F1 ATPase and COX1) or ROS-handling enzymes (SOD2 and catalase). Gastrocnemius was less active than soleus measured as EMG signal and glycogen use yet gastrocnemius displayed larger increases than soleus in phosphorylation of AMPK Thr172, eEF2 Thr56 and ERK 1/2 Thr202/Tyr204 when normalised to the mean relative EMG-signal. In conclusion, proteins with muscle-group restricted expression in mice do not show this pattern in human lower extremity muscle groups. Nonetheless the phosphorylation-response is greater for a number of kinase signalling pathways in human gastrocnemius than soleus at a given activation-intensity. This may be due to the combined subtle effects of a higher type I muscle fiber content and higher training status in soleus compared to gastrocnemius muscle.

The relationship between human skeletal muscle pyruvate dehydrogenase phosphatase activity and muscle aerobic capacity

Pyruvate dehydrogenase (PDH) is a mitochondrial enzyme responsible for regulating the conversion of pyruvate to acetyl-CoA for use in the tricarboxylic acid cycle. PDH is regulated through phosphorylation and inactivation by PDH kinase (PDK) and dephosphorylation and activation by PDH phosphatase (PDP). The effect of endurance training on PDK in humans has been investigated; however, to date no study has examined the effect of endurance training on PDP in humans. Therefore, the purpose of this study was to examine differences in PDP activity and PDP1 protein content in human skeletal muscle across a range of muscle aerobic capacities. This association is important as higher PDP activity and protein content will allow for increased activation of PDH, and carbohydrate oxidation. The main findings of this study were that 1) PDP activity ( r2 = 0.399, P = 0.001) and PDP1 protein expression ( r2 = 0.153, P = 0.039) were positively correlated with citrate synthase (CS) activity as a marker for muscle aerobic capacity; 2) E1α ( r2 = 0.310, P = 0.002) and PDK2 protein ( r2 = 0.229, P =0.012) are positively correlated with muscle CS activity; and 3) although it is the most abundant isoform, PDP1 protein content only explained ∼18% of the variance in PDP activity ( r2 = 0.184, P = 0.033). In addition, PDP1 in combination with E1α explained ∼38% of the variance in PDP activity ( r2 = 0.383, P = 0.005), suggesting that there may be alternative regulatory mechanisms of this enzyme other than protein content. These data suggest that with higher muscle aerobic capacity (CS activity) there is a greater capacity for carbohydrate oxidation (E1α), in concert with higher potential for PDH activation (PDP activity).

Hypoxia inducible factor 1 links fast-patterned muscle activity and fast muscle phenotype in rats

Exercise influences muscle phenotype by the specific pattern of action potentials delivered to the muscle, triggering intracellular signalling pathways. PO2 can be reduced by an order of magnitude in working muscle. In humans, carriers of a hyperactive polymorphism of the transcription factor hypoxia inducible factor 1α (HIF-1α) have 50% more fast fibres, and this polymorphism is prevalent among strength athletes. We have investigated the putative role of HIF-1α in mediating activity changes in muscle.When rat muscles were stimulated with short high frequency bursts of action potentials known to induce a fast muscle phenotype, HIF-1α increased by about 80%. In contrast, a pattern consisting of long low frequency trains known to make fast muscles slow reduced the HIF-1α level of the fast extensor digitorum longus (EDL) muscle by 44%. Nuclear protein extracts from normal EDL contained 2.3-fold more HIF-1α and 4-fold more HIF-1β than the slow soleus muscle, while von-Hippel-Lindau protein was 4.8-fold higher in slow muscles. mRNA displayed a reciprocal pattern; thus FIH-1 mRNA was almost 2-fold higher in fast muscle, while the HIF-1α level was half, and consequently protein/mRNA ratio for HIF-1α was more than 4-fold higher in the fast muscle, suggesting that HIF-1α is strongly suppressed post-transcriptionally in slow muscles.When HIF-1α was overexpressed for 14 days after somatic gene transfer in adult rats, a slow-to-fast transformation was observed, encompassing an increase in fibre cross sectional area, oxidative enzyme activity and myosin heavy chain. The latter was shown to be regulated at the mRNA level in C2C12 myotubes.

The effect of dietary fat content on phospholipid fatty acid profile is muscle fiber type dependent

A high-saturated-fat diet (HFD) induces obesity and insulin resistance (IR). IR has been linked to alterations and increased saturation in the phospholipid composition of skeletal muscles. We aimed to determine whether HFD feeding affects fatty acid (FA) membrane profile in a muscle fiber type-specific manner. We measured phospholipid FAs and expression of FA synthesis genes in oxidative soleus (SOL) and glycolytic extensor digitorum longus (EDL) muscles from rats fed either standard chow (standard laboratory diet, SLD) or a HFD. The HFD increased fat mass, plasma insulin, and leptin levels. Compared with EDL, SOL muscles preferentially accumulated C18 over C16 FAs and n-6 over n-3 polyunsaturated FAs (PUFAs) on either diet. With the HFD, SOL muscles contained more n-9 monounsaturated FAs (MUFAs) and n-6 PUFAs and less n-7 MUFAs and n-3 PUFAs than EDL muscles and had lower unsaturation index, a pattern known to be associated with IR. Stearoyl-CoA desaturase-1 expression was approximately 13-fold greater in EDL than in SOL muscles but did not change with the HFD in either muscle. The expression of Elongase-5 was higher, and that of Elongase-6 (Elovl6) was lower in EDL compared with SOL muscles with both diets. In EDL muscles, the expression of Elovl6 was lower in the HFD than in the SLD. The pattern of FA uptake, expression, and diet-induced changes in FA desaturating and elongating enzymes maintained higher FA unsaturation in EDL muscles. Accordingly, the fiber type composition of skeletal muscles and their distribution may be important in the development and progression of obesity and IR.

Work-related pain in extrinsic finger extensor musculature of instrumentalists is associated with intracellular pH compartmentation during exercise

Background

Although non-specific pain in the upper limb muscles of workers engaged in mild repetitive tasks is a common occupational health problem, much is unknown about the associated structural and biochemical changes. In this study, we compared the muscle energy metabolism of the extrinsic finger extensor musculature in instrumentalists suffering from work-related pain with that of healthy control instrumentalists using non-invasive phosphorus magnetic resonance spectroscopy (³¹P-MRS). We hypothesize that the affected muscles will show alterations related with an impaired energy metabolism.

Methodology/Principal Findings

We studied 19 volunteer instrumentalists (11 subjects with work-related pain affecting the extrinsic finger extensor musculature and 8 healthy controls). We used ³¹P-MRS to find deviations from the expected metabolic response to exercise in phosphocreatine (PCr), inorganic phosphate (Pi), Pi/PCr ratio and intracellular pH kinetics. We observed a reduced finger extensor exercise tolerance in instrumentalists with myalgia, an intracellular pH compartmentation in the form of neutral and acid compartments, as detected by Pi peak splitting in ³¹P-MRS spectra, predominantly in myalgic muscles, and a strong association of this pattern with the condition.

Conclusions/Significance

Work-related pain in the finger extrinsic extensor muscles is associated with intracellular pH compartmentation during exercise, non-invasively detectable by ³¹P-MRS and consistent with the simultaneous energy production by oxidative metabolism and glycolysis. We speculate that a deficit in energy production by oxidative pathways may exist in the affected muscles. Two possible explanations for this would be the partial and/or local reduction of blood supply and the reduction of the muscle oxidative capacity itself.

Phosphocreatine recovery kinetics following low- and high-intensity exercise in human triceps surae and rat posterior hindlimb muscles

Previous studies have suggested the recovery of phosphocreatine (PCr) after exercise is at least second-order in some conditions. Possible explanations for higher-order PCr recovery kinetics include heterogeneity of oxidative capacity among skeletal muscle fibers and ATP production via glycolysis contributing to PCr resynthesis. Ten human subjects (28 +/- 3 yr; mean +/- SE) performed gated plantar flexion exercise bouts consisting of one contraction every 3 s for 90 s (low-intensity) and three contractions every 3 s for 30 s (high-intensity). In a parallel gated study, the sciatic nerve of 15 adult male Sprague-Dawley rats was electrically stimulated at 0.75 Hz for 5.7 min (low intensity) or 5 Hz for 2.1 min (high intensity) to produce isometric contractions of the posterior hindlimb muscles. [(31)P]-MRS was used to measure relative [PCr] changes, and nonnegative least-squares analysis was utilized to resolve the number and magnitude of exponential components of PCr recovery. Following low-intensity exercise, PCr recovered in a monoexponential pattern in humans, but a higher-order pattern was typically observed in rats. Following high-intensity exercise, higher-order PCr recovery kinetics were observed in both humans and rats with an initial fast component (tau < 15 s) resolved in the majority of humans (6/10) and rats (5/8). These findings suggest that heterogeneity of oxidative capacity among skeletal muscle fibers contributes to a higher-order pattern of PCr recovery in rat hindlimb muscles but not in human triceps surae muscles. In addition, the observation of a fast component following high-intensity exercise is consistent with the notion that glycolytic ATP production contributes to PCr resynthesis during the initial stage of recovery.

AMPK and ACC phosphorylation: effect of leptin, muscle fibre type and obesity

Leptin stimulates fatty acid oxidation via the phosphorylation of AMPK (AMP-activated protein kinase) and ACC (acetyl-CoA carboxylase). Obesity is associated with resistance to the effects of leptin. We determined the action of leptin on AMPKalpha and ACCbeta phosphorylation and lipid metabolism in soleus (SOL) and extensor digitorum longus (EDL) muscles from lean and obese Wistar rats after 1 and 100 nM leptin. Both leptin doses stimulated phosphorylation of AMPKalpha and ACCbeta (P<or=0.05) only in EDL muscles from lean animals. Malonyl-CoA levels were decreased in EDL muscles from lean animals after 1 and 100 nM leptin and significantly after 100 nM leptin in obese animals (P<or=0.05). Long-chain fatty acyl-CoA concentrations were decreased in EDL muscles from both phenotypes after 100 nM leptin. AMPK activation by leptin occurred independently of energy-related metabolites. These data demonstrate that the leptin effect on AMPKalpha and ACCbeta is muscle fibre type dependent and fails in diet-induced obesity.

Effects of insulin and transgenic overexpression of UDP-glucose pyrophosphorylase on UDP-glucose and glycogen accumulation in skeletal muscle fibers

UDP-glucose (UDP-Glc) and glycogen levels in skeletal muscle fibers of defined fiber type were measured using microanalytical methods. Infusing rats with insulin increased glycogen in both Type I and Type II fibers. Insulin was without effect on UDP-Glc in Type I fibers but decreased UDP-Glc by 35-40% in Type IIA/D and Type IIB fibers. The reduction in UDP-Glc suggested that UDP-Glc pyrophosphorylase (PPL) activity might limit glycogen synthesis in response to insulin. To explore this possibility, we generated mice overexpressing a UDP-Glc PPL transgene in skeletal muscle. The transgene increased both UDP-Glc PPL activity and levels of UDP-Glc in skeletal muscles by approximately 3-fold. However, overexpression of UDP-Glc PPL was without effect on either the levels of skeletal muscle glycogen or glucose tolerance in vivo. The transgene was also without effect on either control or insulin-stimulated rates of (14)C-glucose incorporation into glycogen in muscles incubated in vitro. The results indicate that UDP-Glc PPL activity is not limiting for glycogen synthesis.

Fatigue of rat hindlimb motor units: Biochemical- physiological associations

Associations between fatigability and biochemical properties within motor unit (MU) types were explored in two hindlimb muscles of the adult rat. Type FF MUs in extensor digitorum longus and type S units in soleus were subjected either to a moderate (type FF) or severe (type S) 6-min, fatigue-inducing stimulation protocol. For both MU types, the range of values for their fatigability was considerably greater than the ranges in the activity levels of three enzymes in the units' constituent muscle fibers (MFs). These enzymes represented major energy-yielding pathways: adenylokinase, for high-energy phosphate metabolism; lactate dehydrogenase, for anaerobic glycolysis; and malate dehydrogenase, for oxidative metabolism. There were also relatively weak associations between the fatigue indices of the MUs and the activity levels of the three enzymes. Thus, this work supports previous conclusions that the force decline exhibited by MUs during electrically evoked contractions depends on both MF biochemistry and other intracellular mechanisms. Electromyographic measurements suggested that these other mechanisms are distal to the intramuscular branches of the axon in type FF units, and distal to excitation-contraction coupling in type S units.

Muscle- and fibre type-specific expression of glucose transporter 4, glycogen synthase and glycogen phosphorylase proteins in human skeletal muscle

The muscle- and fibre type-specific expression of skeletal muscle glucose transporter 4 (GLUT4), glycogen synthase (GS) and glycogen phosphorylase (GP) was investigated in six young male subjects. Single muscle fibres were dissected from vastus lateralis (VL), soleus (SO) and triceps brachii (TB) muscle biopsy samples. On the basis of myosin heavy chain (MHC) expression, fibres were pooled into three groups (MHC I, MHC IIA and MHC IIX) and the GLUT4, GS and GP content of 15-40 pooled fibres determined using SDS-PAGE and immunological detection. In VL, the GLUT4 content in the pooled muscle fibres expressing MHC I was approximately 33% higher ( P<0.05) than in fibres expressing MHC IIA or IIX. There was no difference in GLUT4 content between fibres expressing MHC IIA or IIX, nor were there any differences in GS and GP content between any of the fibre types. In SO, there was no difference in GLUT4, GS and GP between fibres expressing MHC I or IIA. No fibres expressing type IIX were detected. In TB, fibres expressing MHC IIA and IIX had significantly ( P<0.05) more GP (66% and 55 % in MHC IIA and MHCIIX, respectively) than those expressing MHC I, whilst there was no difference in GP between MHC IIA and MHC IIX fibres. The GLUT4 and the GS content was similar in fibres expressing MHC I, IIA and IIX in the TB. Our data directly demonstrate that some proteins, like GLUT4 and GP, are expressed in a fibre type-specific manner in some, but not all, muscles, whilst other proteins, like GS, are not. In human skeletal muscle the GLUT4, GS and GP content thus seems to be related primarily to factors other than the fibre type as defined by the expression of contractile protein. These findings imply that it is not possible to generalize fibre type-dependent protein expression on the basis of biopsies from only one muscle.

Enzymatic alterations in single type IIB skeletal muscle fibers with inactivity and exercise in 12- and 30-month-old rats

The purpose of the present study was to examine the effects of aging, inactivity and weight-bearing exercise on fast-twitch single Type IIB skeletal muscle fibers from the superficial region of the lateral head of the gastrocnemius (Type IIB fibers). Specifically, this study compared the biochemical properties of Type IIB fibers after 7 days of hindlimb unweighting (HU), 7 days of HU with intermittent weight-bearing (HU-Ex), and cage control (C) from adult and aged Fischer 344 Brown Norway F1 Hybrid rats (12- and 30-month old). Biochemical measurements included total lactate dehydrogenase (LDH) and beta-hydroxy-acyl-coenzyme A dehydrogenase activities (BHAD), expressed in nmoles/microg/hr dry weight. Fiber-typing for myosin heavy chain isoform was determined by SDS-PAGE. With age, LDH activity in Type IIB fibers decreased from 52.0 +/- 3.4 nmoles/microg/hr (12-month old) to 39.5 +/- 2.9 nmoles/microg/hr (30-month old). Following HU, LDH activity of single Type IIB fibers increased by 22% (52.0 +/- 3.4 to 66.4 +/- 3.2 nmoles/microg/hr) in the 12-month-old animals, whereas no difference was observed with HU in the Type IIB fibers of the 30-month-old animals. Following HU-Ex, LDH activity of Type IIB fibers in the 12-month-old animals was not significantly different from that of Type IIB fibers from HU animals, whereas a significant increase was observed (38.1 +/- 2.9 to 51.8 +/- 3.1 nmoles/microg/hr) in Type IIB fibers of 30-month-old animals, for HU and HU-Ex, respectively. Analysis of variance revealed an interaction between age and condition, indicating that Type IIB fibers from adult and aged animals have a different biochemical response to inactivity. The enzyme activities for BHAD were not different between the experimental conditions. The results demonstrate that the total LDH enzyme activities of the Type IIB fibers decrease with age, suggesting an age-related shift in the biochemical profile. Further, single skeletal muscle fiber adaptation is age-dependent.

Elevated fasting insulin concentrations associate with impaired insulin signaling in skeletal muscle of healthy subjects independent of obesity

BACKGROUND

Insulin signaling is impaired in the skeletal muscle of obese subjects but whether defects in skeletal muscle insulin signaling also characterize insulin resistance of non-obese individuals is unknown. The detection of insulin signaling defects in muscle biopsies is hampered by the variation of the contaminating non-muscle elements such as blood, connective tissue, fat, and blood vessel structures. Freeze-drying and macroscopic purification of the muscle fibers prior to the analysis might offer a possibility to minimize the analytical variation due to these contaminants.

METHODS

In the present study we first determined whether insulin signaling could be reliably assessed in freeze-dried muscle specimens, which are free of non-muscle contaminants, and then applied this method to the study of insulin signaling in weight-matched insulin-sensitive and insulin-resistant non-diabetic men.

RESULTS

In rat muscle, increases in tyrosine phosphorylation of insulin receptor (IR) and activity of the insulin receptor substrate-1 (IRS-1)-associated phosphatidylinositol (PI) 3-kinase activity by insulin were similar or higher in freeze-dried and purified muscle than wet muscle. Prior to freeze-drying and purification, biopsies of human vastus lateralis muscle contained between 1% and 40% non-muscle contaminants (11+/-3%, mean+/-SEM, n=19). In freeze-dried biopsies of human vastus lateralis muscle taken before and after 30 min of hyperinsulinemia (serum free insulin 61+/-1 mU/l) in 13 non-diabetic men, insulin increased IR tyrosine phosphorylation 1.4-fold (p<0.05) and IRS-1-associated PI 3-kinase activity 1.7-fold (p<0.005). Insulin-stimulated PI 3-kinase activity was significantly inversely correlated with the fasting serum insulin concentration (r=-0.57, p<0.05). When divided according to the median fasting serum insulin concentration, the men with high fasting insulin [HI, n=7, age 44+/-3 years, body mass index (BMI) 25+/-1 kg/m(2)] as compared to the men with low fasting insulin [LI, n=6, age 45+/-3 years (NS), BMI 24+/-1 kg/m(2) (NS)] had lower rates of whole-body glucose uptake (3.4+/-0.4 vs 5.5+/-0.3 mg/kg min, p<0.005), higher fasting plasma glucose concentrations (5.9+/-0.2 vs 5.2+/-0.1 mmol/l, p<0.05), higher fasting serum triglycerides (1.4+/-0.2 vs 0.9+/-0.1 mmol/l, p<0.05) and lower high-density lipoprotein (HDL) cholesterol concentrations (1.3+/-0.1 vs 1.7+/-0.1 mmol/l, p<0.05). Insulin-stimulated IR tyrosine phosphorylation (p<0.05) and IRS-1-associated PI 3-kinase activity (p<0.05) were significantly lower in the HI than the LI group.

CONCLUSIONS

Taken together these data demonstrate that early insulin signaling events can be reliably assessed in freeze-dried human skeletal muscle, and that in vivo insulin resistance and its accompanying features are associated with defects in early insulin signaling events in human skeletal muscle independent of body weight.

Relationship between muscle fibre composition, glucose transporter protein 4 and exercise training: possible consequences in non-insulin-dependent diabetes mellitus

Skeletal muscle is composed of different fibre types, which differ in contractile as well as in metabolic properties. The myosin molecule, which exists in several different isoforms, is of major importance in determining the contractile properties of the muscle cell. The plasticity of skeletal muscle is reflected in this tissue's adaptability to changes in the functional demand. In both rats and humans, a decrease in activity level will in most cases change the muscle fibre composition towards faster myosin isoforms and an increase in activity level (such as seen with exercise training) will induce an increase in slower myosin isoforms. The glucose transporter protein 4 (GLUT4), which is the major insulin regulatable glucose transporter in mammalian skeletal muscle, is found in larger amounts in slow muscle fibres compared with fast muscle fibres. An increase in activity level will increase the GLUT4 protein expression and a decrease in activity level will in most cases decrease GLUT4. Thus, there seems to be some kind of relationship between the muscle fibre type and GLUT4. However, the main factor regulating both the GLUT4 protein expression and the muscle fibre composition seems to be the activity level of the muscle fibre. Patients suffering from non-insulin-dependent diabetes mellitus (NIDDM) are insulin resistant in their skeletal muscles but are generally normal when it comes to skeletal muscle fibre composition and the GLUT4 protein expression. There is good evidence that exercise training beneficially impacts on insulin sensitivity in healthy individuals and in patients with type II diabetes. An increase in the GLUT4 protein expression in skeletal muscle may at least partly explain this effect of training.

Control of glycogen synthesis is shared between glucose transport and glycogen synthase in skeletal muscle fibers

The effects of transgenic overexpression of glycogen synthase in different types of fast-twitch muscle fibers were investigated in individual fibers from the anterior tibialis muscle. Glycogen synthase was severalfold higher in all transgenic fibers, although the extent of overexpression was twofold greater in type IIB fibers. Effects of the transgene on increasing glycogen and phosphorylase and on decreasing UDP-glucose were also more pronounced in type IIB fibers. However, in any grouping of fibers having equivalent malate dehydrogenase activity (an index of oxidative potential), glycogen was higher in the transgenic fibers. Thus increasing synthase is sufficient to enhance glycogen accumulation in all types of fast-twitch fibers. Effects on glucose transport and glycogen synthesis were investigated in experiments in which diaphragm, extensor digitorum longus (EDL), and soleus muscles were incubated in vitro. Transport was not increased by the transgene in any of the muscles. The transgene increased basal [(14)C]glucose into glycogen by 2.5-fold in the EDL, which is composed primarily of IIB fibers. The transgene also enhanced insulin-stimulated glycogen synthesis in the diaphragm and soleus muscles, which are composed of oxidative fiber types. We conclude that increasing glycogen synthase activity increases the rate of glycogen synthesis in both oxidative and glycolytic fibers, implying that the control of glycogen accumulation by insulin in skeletal muscle is distributed between the glucose transport and glycogen synthase steps.

Neuromuscular fatigue during repeated exhaustive submaximal static contractions of knee extensor muscles in endurance-trained, power-trained and untrained men

The neural and muscular changes during fatigue produced in repeated submaximal static contractions of knee extensors were measured. Three groups of differently adapted male subjects (power-trained, endurance-trained and untrained, 15 in each) performed the exercise that consisted of 10 trials of submaximal static contractions at the level of 40% of maximal voluntary contraction (MVC) force till exhaustion with the inter-trial rest intervals of 1 min. MVC force, reaction time and patellar reflex time components before and after the fatiguing exercise and following 5, 10 and 15 min of recovery were recorded. Endurance-trained athletes had a significantly longer holding times for all the 10 trials compared with power-trained athletes and untrained subjects. However, no significant differences in static endurance between power-trained athletes and untrained subjects were noted. The fatigue test significantly prolonged the time between onset of electrical and mechanical activity (electromechanical delay) in voluntary and reflex contractions. The electromechanical delay in voluntary contraction condition for power-trained and untrained subjects and in reflex condition for endurance-trained subjects had not recovered 15 min after cessation of exercise. No significant changes in the central component of visual reaction time (premotor time of MVC) and latency of patellar reflex were noted after fatiguing static exercise. It is concluded, that in this type of exercise the fatigue development may be largely owing to muscle contractile failure.

Substrate and enzyme profile of fast and slow skeletal muscle fibers in rhesus monkeys

Results from the Russian Cosmos program suggest that the rhesus monkey is an excellent model for studying weightlessness-induced changes in muscle function. Consequently, the purpose of this investigation was to establish the resting levels of selected substrate and enzymes in individual slow- and fast-twitch muscle fibers of the rhesus monkey. A second objective was to determine the effect of an 18-day sit in the Spacelab experiment-support primate facility [Experimental System for the Orbiting Primate (ESOP)]. Muscle biopsies of the soleus and medial gastrocnemius muscles were obtained 1 mo before and immediately after an 18-day ESOP sit. The biopsies were freeze-dried, and individual fibers were isolated and assayed for the substrates glycogen and lactate and for the high-energy phosphates ATP and phosphocreatine. Fiber enzyme activity was also determined for the glycolytic enzymes phosphofructokinase and lactate dehydrogenase (LDH) and for the oxidative markers 3-hydroxyacyl-CoA dehydrogenase (beta-OAC) and citrate synthase. Consistent with other species, the fast type II fibers contained higher glycogen content than did the slow type I fibers. The ESOP sit had no significant effects on the metabolic profile of the slow fibers of either muscle or the fast fibers of the soleus. However, the fast gastrocnemius fibers showed a significant decline in phosphocreatine and an increase in lactate. Also, similar to other species, the fast fibers contained significantly higher LDH activities and lower 3-hydroxyacyl-CoA dehydrogenase activities. For the muscle enzymes, the quantitatively most important effect of the ESOP sit occurred with LDH where activities increased in all fiber types postsit except the slow type I fiber of the medial gastrocnemius.

Determinants of oxygen uptake. Implications for exercise testing

For exercise modalities such as cycling which recruit a substantial muscle mass, muscle oxygen uptake (VO2) is the primary determinant of pulmonary VO2. Indeed, the kinetic complexities of pulmonary VO2 associated with exercise onset and the non-steady state of heavy (> lactate threshold) and severe [> asymptote of power-time relationship for high intensity exercise (W)] exercise reproduce with close temporal and quantitative fidelity those occurring across the exercising muscles. For moderate (< lactate threshold) exercise and also rapidly incremental work tests, pulmonary (and muscle) VO2 increases as a linear function of work rate (approximately equal to 9 to 11 ml O2/W/min) in accordance with theoretical determinations of muscle efficiency (approximately equal to 30%). In contrast, for constant load exercise performed in the heavy and severe domains, a slow component of the VO2 response is manifest and pulmonary and muscle VO2 increase as a function of time as well as work rate beyond the initial transient associated with exercise onset. In these instances, muscle efficiency is reduced as the VO2 cost per unit of work becomes elevated, and in the severe domain, this VO2 slow component drives VO2 to its maximum and fatigue ensues rapidly. At pulmonary maximum oxygen uptake (VO2max) during cycling, the maximal cardiac output places a low limiting ceiling on peak muscle blood flow, O2 delivery and thus muscle VO2. However, when the exercise is designed to recruit a smaller muscle mass (e.g. leg extensors, 2 to 3kg), mass-specific muscle blood flow and VO2 at maximal exercise are 2 to 3 times higher than during conventional cycling. consequently, for any exercise which recruits more than approximately equal to 5 to 6kg of muscle at pulmonary VO2max, there exists a mitochondrial or VO2 reserve capacity within the exercising muscles which cannot be accessed due to oxygen delivery limitations. The implications of these latter findings relate to the design of exercise tests. Specifically, if the purpose of exercise testing is to evaluate the oxidative capacity of a small muscle mass (< 5 to 6kg), the testing procedure should be designed to restrict the exercise to those muscles so that a central (cardiac output, muscle O2 delivery) limitation is not invoked. It must be appreciated that exercise which recruits a greater muscle mass will not stress the maximum mass-specific muscle blood flow and VO2 but rather the integration of central (cardiorespiratory) and peripheral (muscle O2 diffusing capacity) limitations.

Increased glycogen accumulation in transgenic mice overexpressing glycogen synthase in skeletal muscle

To investigate the role of glycogen synthase in controlling glycogen accumulation, we generated three lines of transgenic mice in which the enzyme was overexpressed in skeletal muscle by using promoter-enhancer elements derived from the mouse muscle creatine kinase gene. In all three lines, expression was highest in muscles composed primarily of fast-twitch fibers, such as the gastrocnemius and anterior tibialis. In these muscles, glycogen synthase activity was increased by as much as 10-fold, with concomitant increases (up to 5-fold) in the glycogen content. The uridine diphosphoglucose concentrations were markedly decreased, consistent with the increase in glycogen synthase activity. Levels of glycogen phosphorylase in these muscles increased (up to 3-fold), whereas the amount of the insulin-sensitive glucose transporter 4 either remained unchanged or decreased. The observation that increasing glycogen synthase enhances glycogen accumulation supports the conclusion that the activation of glycogen synthase, as well as glucose transport, contributes to the accumulation of glycogen in response to insulin in skeletal muscle.

Muscle glycogen resynthesis after short term, high intensity exercise and resistance exercise

Typical rates of muscle glycogen resynthesis after short term, high intensity exercise (15.1 to 33.6 mmol/kg/h) are much higher than glycogen resynthesis rates following prolonged exercise (approximately 2 mmol/kg/h), even when optimal amounts of oral carbohydrate are supplied (approximately mmol/kg/h). Several factors differ during post-exercise recovery from short term, high intensity exercise compared with prolonged exercise. The extremely fast rate of muscle glycogen resynthesis following short term, high intensity exercise may originate from these differences. First, peak blood glucose levels range from 6.6 to 8.9 mmol/L during recovery from short term, high intensity exercise. This is markedly higher than the blood glucose values of 2 to 3.4 mmol/L after prolonged exercise. In response to this elevation in plasma glucose levels, insulin levels increase to approximately 60 microU/ml, a 2-fold increase over resting values. Both glucose and insulin regulate glycogen synthase activity, and higher levels of them improve muscle glycogen synthesis. Secondly, high intensity exercise produces high levels of glycolytic intermediates in muscle, as well as high lactate levels ([La]) in muscle and blood. Finally, fast-twitch glycolytic muscle fibres are more heavily used in short term, high intensity exercise. This promotes greater glycogen depletion in the fast-twitch fibres, which have a higher level of glycogen synthase activity than slow-twitch fibres. While the exact contribution of each of these factors is unknown, they may act in combination to stimulate rapid muscle glycogen resynthesis rates. Muscle glycogen resynthesis rates following resistance exercise (1.3 to 11.1 mmol/kg/h) are slower than the rates observed after short term, high intensity exercise. This may be caused by slightly lower muscle and blood [La] after resistance exercise. In addition, a greater eccentric component in the resistance exercise may cause some interference with glycogen resynthesis.

Alteration of regulatory enzyme activities in fast-twitch and slow-twitch muscles and muscle fibres in low-intensity endurance-trained rats

The effect of progressive, low-intensity endurance training on regulatory enzyme activities in slow-twitch (ST) and fast-twitch (FT) muscle fibres was studied in 32 rats. Of those rats 16 were trained on a treadmill at a running speed of 10 m.min-1 5 days a week over an 8-week period. Running time was progressively increased from 15 min to 2 h.day-1. Of the rats 4 trained and 4 sedentary rats were also subjected to acute exhausting exercise. Enzyme activities of phosphofructokinase 1 (PFK1) from glycolysis, alpha-ketoglutarate dehydrogenase (alpha-KGDH) from the Krebs cycle and carnitine palmitoyltransferase (CPT I and II) from fatty acid metabolism in soleus, tibialis anterior and gastrocnemius muscles were measured in trained and sedentary rats. Enzyme activities of individual ST and FT fibres were measured from the freeze-dried gastrocnemius muscle of 8 trained and 8 sedentary rats. In the sedentary rats the activity of PFK1 in tibialis anterior and soleus muscles was 141% and 41% of the activity in gastrocnemius muscle, respectively. The activity of alpha-KGDH in tibialis anterior and soleus muscles was 164% and 278% of the activity in gastrocnemius muscle, respectively. The activity of CPT I in tibialis anterior and gastrocnemius muscles were at the same level, but in soleus muscle the activity was 127% of that in mixed muscle. Endurance training increased enzyme activities of alpha-KGDH and CPT I significantly (P < 0.05) in gastrocnemius muscle but not in soleus or tibialis anterior muscle.(ABSTRACT TRUNCATED AT 250 WORDS)

Nerve-dependent recovery of metabolic pathways in regenerating soleus muscles

The metabolic recovery potential of muscle was studied in regenerating soleus muscles of young adult rats. Degeneration was induced by subfascial injection of a myotoxic snake venom. After regeneration for selected periods up to 2 weeks, samples of whole muscle were analysed for hexokinase (EC 2.7.1.1), phosphofructokinase (EC 2.7.1.11), lactate dehydrogenase (EC 1.1.11.27), adenylokinase (EC 2.7.4.3), creatine kinase (EC 2.7.3.2), malate dehydrogenase (EC 1.1.11.37), citrate synthase (EC 4.1.3.7) and beta-hydroxyacyl CoA dehydrogenase (EC 1.1.1.35). Lactate dehydrogenase, adenylokinase, malate dehydrogenase and beta-hydroxyacyl CoA dehydrogenase were also measured in individual fibres of muscle regenerating up to 4 weeks. We found that in the presence of nerve there was complete recovery of muscle metabolic capacity. However, there were differences in the rate of recovery of the activity of enzymes belonging to different energy-generating pathways. Lactate dehydrogenase, an enzyme representing glycolytic metabolism, reached normal activity immediately upon myofibre formation, only 3 days after venom injection, while oxidative enzymes required a week or more to reach normal activity levels. The delay in oxidative enzyme recovery coincided with physiological parameters of reinnervation. Therefore, to further test the role of nerve on the metabolic recovery process, muscle regeneration was studied following venom-induced degeneration coupled with denervation. In the absence of innervation, most enzymes failed to recover to normal activity levels. Lactate dehydrogenase was the only enzyme to achieve normal levels, and it did so as rapidly as in innervated-regenerating soleus muscles. The remainder of the glycolytic enzymes and the high energy phosphate enzymes recovered only partially. Oxidative enzymes showed no recovery and were severely reduced in the absence of reinnervation.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of tetrodotoxin-induced neural inactivation on single muscle fiber metabolic enzymes

Selected enzymes were measured in mixed-fiber bundles and individual fibers from rat plantaris (PL) and soleus (Sol) muscles that had undergone either 2 wk of tetrodotoxin (TTX) inactivation of the sciatic nerve, a sham operation, or were contralateral to the TTX limb. TTX disuse caused severe wasting of PL (46%) and Sol (26%) muscles and of single fibers (50% and 40%, respectively). TTX PL and Sol also had reduced (50%) glycogen content. In TTX, PL, and Sol macro samples and single fibers, the activities (mol.h-1.kg dry wt-1) of hexokinase, glycogen phosphorylase, and lactate dehydrogenase were higher, lower, and unchanged, respectively, compared with controls. Single-fiber data showed that these changes occurred in all fibers. In TTX PL macro samples, activities of glycerol-3-phosphate dehydrogenase (GPDH), pyruvate kinase (PK), malate dehydrogenase (MDH), citrate synthase (CS), beta-hydroxyacyl-CoA dehydrogenase (BOAC), and thiolase were, or tended to be, lower. Single-fiber data showed a disappearance of high-oxidative moderate glycolytic fibers (i.e., usually fast-twitch oxidative in control) and the appearance of more fibers with a metabolic enzyme profile approaching that of control slow-oxidative fibers. In TTX Sol macro samples, GPDH and PK tended to be higher, and thiolase, BOAC, CS, and MDH lower. Single-fiber data corroborated these findings and suggested the appearance of fast fibers with downregulated oxidative enzyme profiles. Our results suggest that neuromuscular activity is a major, but not the sole, determinant of the size and metabolic heterogeneity that exists in muscle cells.

Metabolic capacity of muscle fibers from high-altitude natives

We evaluate the effects of chronic hypoxia on the metabolic phenotype of the muscle fiber types of humans. The subjects were three Quechua natives residing in the Peruvian Andes at an altitude greater than 3300 m, and three lowlanders from below 700 m. Biopsy specimens were obtained from the vastus lateralis muscles of volunteers. Muscle fibers were identified histochemically as type 1 (oxidative), 2a (oxidative-glycolytic) or 2b (glycolytic). The relative contribution of each fiber type to the total cross-sectional area of each biopsy sample was determined. In individual fibers, the activities of malate dehydrogenase (MDH, citric acid cycle), lactate dehydrogenase (LDH, glycolysis) and adenylokinase (high-energy phosphate) were quantified. The cross-sectional area of the muscle occupied by each fiber type is comparable between Quechuas and lowlanders. Type 1 fibers are the only fiber type to demonstrate statistically significant (P < or = 0.05) differences in enzyme activities between Quechaus and lowlanders. MDH activity is, on average, 19.6% less (P < or = 0.0001) and LDH activity 28.1% more (P < or = 0.0001) in the type 1 fibers of the Quechuas. Chronic hypoxia appears to produce a shift from oxidative to glycolytic metabolism in those fibers which are typically the most aerobic in human muscle.

Altered malate dehydrogenase activity in nucleus magnocellularis of the chicken following cochlea removal

The metabolism of second order auditory neurons in nucleus magnocellularis (NM) in the chick brainstem can be profoundly altered when excitatory input from the cochlea is removed. Within hours of cochlea removal, NM neurons show an increase in histochemical staining for the Kreb's cycle enzyme succinate dehydrogenase (SDH), followed in several days by decreases in SDH staining. We examined the activity of another Kreb's cycle enzyme, malate dehydrogenase (MDH) using a histochemical stain and a sensitive quantitative biochemical assay for comparison. We found changes in MDH staining similar in time course and magnitude to those of SDH; within 4 h of cochlea removal, MDH activity increases in ipsilateral NM neurons. By 9 days after cochlea removal MDH activity decreases, although not to the same degree as seen with SDH. Biochemical measurements of MDH activity also showed an early increase in activity in ipsilateral NM, followed at 9 days survival by a decrease in activity. Biochemical measurements of the activity of other enzymes in NM may be useful in further defining the metabolic consequences of deafferentation.

Spatial arrangement and metabolic capacity of fiber types in self-reinnervated cat muscle

The recovery potential of skeletal muscle was explored by examining cat muscle between 10 and 33 mo after complete transection and immediate surgical reunion of its own nerve. Biochemical analysis of single muscle fibers showed that the activities of key enzymes in energy metabolism (malate and lactate dehydrogenase and adenylokinase) were similar to normal for their respective fiber types, suggesting that incomplete recovery of the ability to sustain submaximal contraction in reinnervated muscles (T.C. Cope, C.B. Webb, and B.R. Botterman. J. Neurophysiol. 65: 648-656, 1991) is explained in some other way. Two independent statistical procedures for assessing the randomness of adjacencies of histochemically identified fiber types showed type grouping in some areas, but there were also many regions with randomly distributed fiber types. These findings demonstrate the potential for substantial recovery of both energy metabolism and dispersion of fiber types after self-reinnervation.

Soma diameter and oxidative enzyme activity of identified alpha-motoneurons: application of a retrograde fluorescent neuronal tracer

Using a fluorescent compound as a retrograde neuronal tracer for the identification of a specific motoneuron pool, we examined the soma diameter and oxidative enzyme activity of alpha-motoneurons innervating the extensor digitorum longus (EDL) and the soleus (Sol) muscles in the rat. An inverse relationship between soma diameter and oxidative enzyme activity was demonstrated in the EDL, while there was no significant relationship between these variables in the Sol. These results suggest that an inverse relationship between soma diameter and oxidative enzyme activity cannot always be demonstrated in all of the motoneuron pools.

Metabolic and contractile uniformity of isolated motor unit fibres of snake muscle

1. Motor units in the thin transversus abdominis muscle of the garter snake were identified and physiologically characterized in the living state. Motor unit fibres, and fibres chosen randomly to serve as controls, were subsequently excised and subjected to biochemical analyses. 2. The metabolic capacity of fibres was assessed by measuring activities of three enzymes, each representing a different metabolic pathway. The microchemical enzyme assays were performed using enzyme extraction preparations of whole single fibres. 3. Metabolic capacity ranged widely among the muscle's entire fibre population, even among fibres of the same type. In contrast, enzyme activities of twitch fibres belonging to individual motor units were, within analytical error, identical. 4. Twitch contraction times of individual fibres within one motor unit were similar, compared to a wide range of contraction times observed among fibres of the same type but belonging to different motor units. 5. When several motor units were studied in one muscle, a systematic relationship was observed among motor unit tension, enzymatic profile and contraction time. As motor unit tension increased, fibres exhibited greater capacities for glycolytic and high-energy phosphate metabolism, diminished capacity for oxidative metabolism, and faster twitch contraction times. 6. Given the great diversity of metabolic and contractile properties exhibited within the fibre population, the uniformity of such properties within motor units indicates that neural influence dominates over other extrinsic factors present in the microenvironment of the muscle fibres.

Metabolic capacity and myosin expression in single muscle fibres of the garter snake

1. The transversus abdominis muscle of the garter snake contains fibres of three types: tonic (T), slower twitch (S) and faster twitch (F). Fibre types can be determined by anatomical criteria in living preparations. Individual fibres identified as T, S or F were excised from the muscle and subdivided for two types of biochemical examination. Enzymes of energy metabolism were assayed using quantitative microfluorometric methods. Myosin heavy chain composition was determined by gel electrophoresis. In separate experiments, twitch time-to-peaks of F and S fibres were measured to assess the range of contraction times present within the muscle's twitch fibre population. 2. Metabolic subgroups of fibres were delineated by the relative activities of adenylokinase (AK), lactate dehydrogenase (LDH) and beta-hydroxyacyl-CoA-dehydrogenase (beta OAC). The metabolic subgroups corresponded to the anatomical fibre types. Type F fibres had high levels of enzymes associated with glycolytic (LDH) and high-energy phosphate (AK) metabolism. Type T fibres had high levels of the oxidative enzyme beta OAC. Type S fibres had both types of enzyme activity in intermediate and variable amounts. 3. Three myosin heavy chain isoforms were present in the muscle. Type F and type T fibres each expressed a single isoform, denoted F and T respectively. Type S fibres expressed significant quantities of two isoforms: an isoform unique to this fibre type (denoted S) and the F isoform. 4. Electrophoretic mobility and antibody reactivity of the F myosin heavy chain isoform resembled that of mammalian fast-twitch myosin. By the same criteria, the T isoform resembled mammalian slow-twitch myosin. The S isoform exhibited intermediate characteristics: its antibody reactivity was similar to mammalian fast-twitch myosin, but its electrophoretic mobility was that of mammalian slow-twitch myosin. 5. Based on whole-muscle analysis, two myosin alkali light chains, denoted ALC1 and ALC2, and one myosin regulatory light chain were present. Gel patterns suggested that ALC1 and ALC2 exist as both homodimers and heterodimers. 6. The population of type S fibres within a given muscle exhibited a much wider range of twitch contraction times than did the population of type F fibres. Diversity of contractile properties among type S fibres may result, in part, from differential co-expression of two myosin heavy chain isoforms, together with highly variable ratios of enzymes from two major metabolic pathways. 7. The clear biochemical distinction among fibre types indicates that each type possesses a unique and limited range of physiological properties.(ABSTRACT TRUNCATED AT 400 WORDS)

Coordinate enzymatic activity of beta-oxidation and purine nucleotide cycle in a diversity of muscle and other organs of rat

1. Most mammalian muscles consist of a mixture of different muscle fiber types. 2. We analyzed various muscles with different percentages of slow and fast fibers in addition to other organs of rat for enzyme activities of beta-oxidation and the purine nucleotide cycle (PNC). 3. According to the content of slow-twitch fibers all enzymes of beta-oxidation were high in activity whereas enzymes of the purine nucleotide cycle were low. 4. Amongst all enzymes of beta-oxidation, crotonase showed the highest activity. 5. In heart muscle, enzyme activities of beta-oxidation were even higher than in m. soleus which consists almost exclusively of slow-twitch type I fibers. 6. Measurements of all three enzymes involved in the purine nucleotide cycle revealed high activities in muscles predominantly composed of fast-twitch fibers. 7. It was always adenylate deaminase which revealed the highest activity. 8. Heart muscle showed low activities for enzymes of PNC.

Enzymes of transmitter and energy metabolism in cat middle ear muscles

Activities of the enzymes choline acetyltransferase (ChAT) and acetylcholinesterase (AChE), which metabolize the neuromuscular transmitter acetylcholine, and malate and lactate dehydrogenase (MDH and LDH), enzymes of oxidative and glycolytic energy metabolism, respectively, were measured in the middle ear muscles of the cat. For comparison, the same enzyme activities were measured in extraocular muscle tissue and in three hindlimb muscles rich in either slow oxidative (soleus), fast glycolytic (white part of vastus lateralis), or fast oxidative glycolytic (plantaris) muscle fibers. ChAT and AChE activities were much higher in middle ear muscles than in hindlimb muscles, consistent with a denser neuromuscular innervation, as in extraocular muscles. By contrast, MDH and LDH activities were remarkably low in the middle ear muscles, lower than in any of the hindlimb muscles or the extraocular muscles. Denervation of the stapedius muscle by peripheral transection of the facial nerve resulted in decreases in all four enzyme activities without associated changes in the tensor tympani. Surgical ablation of the peripheral facial nerve supply to the stapedius muscle appears to be a feasible option for producing its denervation. The results suggest some rather specialized chemical characteristics for the middle ear muscles.

Enzymes of transmitter and energy metabolism in rat middle ear and extraocular muscles

To further investigate the peculiar characteristics of the middle ear and extraocular muscles, compared to the extensively studied skeletal muscles of the limbs, activities of enzymes of transmitter and energy metabolism were measured in homogenates of these muscles from albino and pigmented rats. These activities were compared to those for a masticatory muscle and for three hindlimb muscles chosen for their preponderance of either slow oxidative, fast glycolytic, or fast oxidative glycolytic fibers. Activities of the neuromuscular transmitter enzymes choline acetyltransferase and acetylcholinesterase were relatively very high in the extraocular and middle ear muscles. The activity of malate dehydrogenase, an enzyme of oxidative energy metabolism, was as high in the extraocular, masticatory and stapedius muscles as in the oxidative hindlimb muscles, but was lower in tensor tympani. The activity of lactate dehydrogenase, an enzyme of glycolytic energy metabolism, was remarkably low in both middle ear muscles. The results are consistent with high innervation density in the extraocular and middle ear muscles, and highly oxidative metabolism in the extraocular and stapedius muscles. Metabolic differences between the stapedius and tensor tympani suggest a relatively more active role for the former in the function of the rat middle ear.

Polarized 86Rb+ effluxes in primary cultures of rabbit kidney proximal cells: role of calcium and hypotonicity

Isolated proximal cells from rabbit kidney were seeded on collagen-coated permeable supports. After 8 days, the cultured cells became organized as a confluent monolayer. The proximal origin of the monolayer was confirmed by enzymatic, immunological, electrical and electron microscopical studies. The epithelia exhibited a morphological polarity that allowed for measurements of effluxes across the apical or the basolateral membranes. 86Rb was used as an isotopic tracer to indicate potassium movements. The 86Rb+ efflux across the basolateral face was 1.93-times that across the apical face, and both effluxes were pH dependent. Apical and basolateral 86Rb+ effluxes increased when the Ca2+ ionophore ionomycin (3 microM) was applied and when monolayers were exposed to a hypotonic medium. A pharmacological study revealed that BaCl2 (5 mM), tetraethylammonium (TEA, 20 mM) and Leiurus quinquestriatus hebraeus scorpion venom (from which charybdotoxin is extracted) abolished both ionomycin and hypotonically-stimulated effluxes, whereas apamin had no significant effect on the hypotonically-stimulated 86Rb+ efflux. This stimulated efflux was also abolished when monolayers were preincubated with pertussis toxin, but did not decrease in a Ca2(+)-free medium.

Ultrastructural and metabolic characteristics of single muscle fibres belonging to the same type in various muscles in rats

Single muscle fibres from soleus (SOL) as well as extensor digitorum longus (EDL) muscles from Wistar male rats in relaxing solution were divided into three types by their histochemical features--slow-twitch oxidative (SO), fast-twitch oxidative glycolytic (FOG), or fast-twitch glycolytic (FG) fibres. The relationship between ultrastructural profiles (mitochondrial volume, number, and Z-line width) and metabolic profiles (glycolytic and oxidative enzymes' activity) were analyzed using the same types of fibres dissected from different SOL and EDL muscles using stereological and biochemical techniques. The Z-line width is specialized in different fibre types. Fast-twitch (FG and FOG) fibres have narrow Z-line width compared to slow-twitch (SO) fibre in SOL and EDL muscles. A significant difference was observed between SOL muscle SO and FOG fibres and EDL muscle SO and FOG fibres. All glycolytic (lactate dehydrogenase (LDH), phosphofructokinase (PFK), pyruvate kinase (PK) and creatine kinase (CK) activities in FOG fibres from EDL muscles were significantly higher (p less than 0.01) than those dissected from SOL muscles. The oxidative enzyme (succinate dehydrogenase (SDH) and malate dehydrogenase (MDH) activity in SO and FOG fibres dissected from SOL muscle were significantly higher (p less than 0.01) than those dissected from EDL muscles. Mitochondrial volume and number in SO fibres dissected from SOL muscle were significantly higher (volume, p less than 0.01, number, p less than 0.01) than those dissected from EDL muscles. A significant difference was not observed in mitochondrial volume of FOG fibres between SOL and EDL muscles.(ABSTRACT TRUNCATED AT 250 WORDS)

Different metabolic responses to exercise training programmes in single rat muscle fibres

The aim of this report is to elucidate the effects of exercise training on metabolic properties of different muscle fibre types of the rat hindlimb. Single muscle fibres were dissected from soleus (SOL) or extensor digitorum longus (EDL) muscles of Wistar strain male rats trained on a treadmill for 16 weeks. Each fibre was typed histochemically (SO, slow-twitch oxidative; FOG, fast-twitch oxidative glycolytic; FG, fast-twitch glycolytic). Then glycolytic and oxidative enzymes (CK, LDH, PFK, PK, SDH, and MDH) activities were measured biochemically. Slow-type fibres (SO) were hypertrophied following endurance training and fast-twitch fibres (FOG and FG) were hypertrophied following sprint training. In EDL muscles the distribution of the slow-type fibres was reduced following the sprint training. The activity of glycolytic enzymes increased significantly in the fast-type fibres (FOG and FG) following sprint training, while oxidative enzymes activities increased in both fast (FOG and FG) and slow (SO) muscle fibres following the endurance training. Neither glycolytic nor oxidative enzymes' activities always increased equally in all types of fibre following exercise training. Consequently, the metabolic profiles in each type of single muscle fibre were affected differently by different intensities of exercise training. These results suggest that the functional (enzymes activity) and structural (muscle fibre hypertrophy) changes of skeletal muscle fibre following exercise training appeared gradually, and would be controlled by different factors.

Abnormal expression of glycogen phosphorylase genes in regenerated muscle

Physiological and molecular biological properties of free, orthotopic grafts of rat extensor digitorum longus (EDL) muscle were determined at 28-, 42-, and 76-days postgraft. cDNA probes for the rat fetal (B), liver (L), and muscle (M) isozymes of glycogen phosphorylase were used to assay isozyme mRNA levels. Regenerating muscle grafts did not express nonmuscle phosphorylase isozymes in vivo in contrast to primary rat skeletal muscle explants in vitro. Low levels of M-phosphorylase mRNA were present at all stages of regeneration in the grafts. However, M-phosphorylase mRNA levels and activity increased markedly and nonuniformly in a subset of functionally and morphologically stabilized regenerated muscle fibers between 42- and 76-days postgraft. Biochemical, physiological, and histochemical characterization of the stabilized grafts demonstrated that all fibers present were innervated and indicated that innervation might be a necessary, but not sufficient, condition for the increase in M-phosphorylase expression. The nonuniform appearance of phosphorylase activity suggests that a differential activity profile imposed on muscle fibers by their motoneuron may govern M-phosphorylase gene expression.

Quantitative analysis of histochemical and immunohistochemical reactions in skeletal muscle fibres of Rana and Xenopus

Intensities of histochemical and immunohistochemical reactions in muscle fibres of Rana and Xenopus have been estimated microphotometrically, and the data from serial sections statically analysed. Quantitative validities of reactions and measurements have also been assessed against independent published evidence. It is concluded that NADH-tetrazolium reductase overestimates tonic-fibre aerobic capacities and the actomyosin ATPase reaction overestimates their contraction speeds. However, it appears that succinate dehydrogenase, despite being a near-equilibrium enzyme of particulate distribution, indicates the relative aerobic capacities of fibres with acceptable accuracy when lightly reacted. Capacities for aerobic and anaerobic metabolism are positively correlated over all types of fibre (r typically approximately 0.6 for 200 fibres), perhaps as an adaptation to environmental hypoxia. Multivariate clusters (indicating fibre types) have been sought, using Ward's method with optimizing procedures (iterative relocation and multivariate-normal modelling). Cluster analysis confirms the subjective identifications of two 'slow/tonic' types in Xenopus (labelled T5 and S4) but of only one (T5) in Rana. Division of the 'fast family' twitch fibres into three types (F1-F3) in both genera, with metabolic capacity related inversely to apparent shortening velocity, is highly supportable by objective criteria. However, statistically significant subdivisions also present themselves. Rana F2 and Xenopus F1 clusters can be bisected according to metabolic capacity; and Xenopus F2 fibres fall into three subtypes reflecting different isomyosin contents. In the different types of twitch fibre, ratios of myofibrillar ATP consumption rate to aerobic capacity increase up to 30-fold with contraction speed, but anaerobic/aerobic ratios do so only 5-fold.

Distribution and developmental transition of phosphoglycerate mutase and creatine phosphokinase isozymes in rat muscles of different fiber-type composition

Phosphoglycerate mutase and creatine phosphokinase have in mammals three isozymes (types MM, MB and BB) with similar tissue distribution and developmental transition in muscle cells. To assess whether the phenotype and the developmental switch of these isozymes differ in the diverse types of muscle fibers, the enzymatic activities and the isozyme patterns, analyzed by cellulose acetate electrophoresis, have been determined in rat soleus, extensor digitorum longus and gastrocnemius muscles during postnatal development. Both phosphoglycerate mutase and creatine phosphokinase activity increased in the three muscles, the increase in extensor digitorum longus and gastrocnemius being higher than in soleus. For the two enzymes the increase in activity was due to the progressive increment of the muscle-specific forms. It is concluded that whereas phosphoglycerate mutase and creatine phosphokinase type-B subunits are present at similar levels in both type I and type II muscle fibers, phosphoglycerate mutase and creatine phosphokinase type-M subunits exhibit much higher levels in type II fibers.

Metabolic fiber types of snake transversus abdominis muscle

Fibers of the garter snake transversus abdominis muscle fall into three classes according to contraction speed: faster and slower twitch and tonic. To determine the relationship between these physiologically determined classes and established mammalian fiber types, individual fibers were assayed for key enzymes representing the major energy-generating pathways in vertebrate muscle. Five such enzymes were examined: lactate dehydrogenase, malate dehydrogenase, adenylokinase, fumarate hydratase, and beta-hydroxyacyl-CoA dehydrogenase. The muscle contained three principal metabolic fiber types. Fast-contracting twitch fibers had low-oxidative but high-glycolytic capacity and therefore resembled mammalian-type fast-twitch glycolytic (FG) fibers. Slower twitch fibers were high oxidative-high glycolytic, similar to mammalian-type fast-twitch, oxidative, glycolytic (FOG) fibers. Tonic fibers were high oxidative-low glycolytic; this metabolic profile is characteristic of type slow-twitch oxidative (SO) fibers in mammals. Activity of the enzyme adenylokinase, which in mammals correlates with contraction speed and myosin adenosine triphosphatase (ATPase) activity, separated these reptilian fibers into three groups that are similar but not identical to those delineated by oxidative and glycolytic enzymes. Adenylokinase and beta-hydroxyacyl-CoA dehydrogenase showed the widest range of activities in snake muscle and, therefore, the greatest ability to discriminate fiber types.

Single muscle fiber enzyme shifts with hindlimb suspension and immobilization

The purpose of this investigation was to determine how models of weightlessness, hindlimb suspension (HS), and hindlimb immobilization (HI) affect the metabolic enzyme profile in the slow oxidative (SO), fast oxidative glycolytic (FOG), and fast glycolytic (FG) fibers of rat hindlimb. After 1, 2, or 4 wk of HS or HI, single fibers were isolated from freeze-dried soleus and gastrocnemius muscles; a small section of each fiber was run on sodium dodecyl sulfate-polyacrylamide gel electrophoresis gels to identify fiber type, and the remaining piece was assayed for either lactate dehydrogenase (LDH) and citrate synthase (CS) or phosphofructokinase (PFK) and beta-hydroxyacyl-CoA dehydrogenase (beta-OH-acyl-CoA). Two weeks of HS induced an almost twofold increase in the activity of CS (2.13 +/- 0.13 vs. 3.60 +/- 0.26 mol.kg dry wt-1.h-1) in the SO fiber of the soleus, and the activity stayed high at 4 wk. Although the FOG fiber had significantly higher CS activity (3.85 +/- 0.29) than either the SO or FG (1.59 +/- 0.16 mol.kg dry wt-1.h-1) fiber, neither fast fiber type was altered by HS. The glycolytic enzymes LDH and PFK were both elevated in the SO fiber after HS. The increase in LDH occurred by 1 wk (14.80 +/- 1.51 vs. 8.83 +/- 0.78), whereas the activity of PFK was not significantly changed until 4 wk (1.16 +/- 0.13 vs. 0.68 +/- 0.05 mol.kg dry wt-1.h-1). The control FG fiber had the highest LDH (44.30 +/- 2.29) and PFK (2.40 +/- 0.16) activities, followed by the FOG fiber (LDH, 34.10 +/- 2.83; PFK, 1.62 +/- 0.17 mol.kg dry wt-1.h-1); however, the activities of these glycolytic enzymes in the fast fiber types were unaltered by HS. The activity of beta-OH-acyl-CoA was not affected by HS in either the slow or fast fiber types. HI showed qualitatively similar changes to those observed with HS; however, the enzyme shifts developed with a slower time course. In conclusion, both HS and HI shifted the SO fiber enzyme pattern toward that of the control FOG fiber; however, a complete conversion from the SO to FOG fiber did not occur within the 4-wk treatment period.

Energy metabolism of the untrained muscle of elite runners as observed by 31P magnetic resonance spectroscopy: evidence suggesting a genetic endowment for endurance exercise

The purpose of this study was to investigate whether genetically determined properties of muscle metabolism contribute to the exceptional physical endurance of world-class distance runners. ATP, phosphocreatine, inorganic phosphate, and pH were quantitatively determined by 31P nuclear magnetic resonance spectroscopy in the wrist flexor muscles of elite long-distance runners and sedentary control subjects. These muscles had not been exposed to any specific program of exercise training in either group of subjects. The "untrained" muscles were examined at rest, during two cycles of three grades of exercise, and in recovery. The flexor muscles of the athletes had higher concentrations of phosphocreatine and ATP than did those of the control subjects at rest and during exercise. The athletes' muscles possessed a higher capacity for generation of ATP by oxidative metabolism than did control subjects' muscles according to the following criteria: (i) high force output, 60% of maximum voluntary contraction, was more easily reached and better maintained in both exercise cycles; (ii) the ratio of inorganic phosphate to phosphocreatine rose less during exercise and recovered faster in the postexercise period; (iii) there was no loss of adenine nucleotides or total phosphate from the athletes' muscles but significant losses from the control subjects' muscles; and (iv) the pH decreased no more than 0.1 unit in the athletes' muscles during exercise, attesting to a relatively slow glycolysis and/or a rapid oxidation of lactate. In the muscles of the control subjects, on the other hand, the pH decreased nearly 0.4 unit early in the first exercise cycle, indicating a relatively fast glycolysis and/or slower oxidation of lactate. In the second exercise cycle, the pH returned to near normal in the control subjects' muscles, reflecting diminished lactate formation because of glycogen depletion and lactate washout by the high blood flow induced by exercise. By the end of the exercise program, the maximum voluntary contractile force for the control subjects had declined to less than 60% of the initial value. This decline could be explained best by exhaustion of the glycolytic contribution to muscle contraction. Therefore, the residual maximum strength provided a measure of the oxidative capacity to support contraction, as is discussed. In conclusion, we suggest that a greater oxidative capacity relative to glycolytic capacity for support of contraction in untrained muscle of world-class runners reflects a genetic endowment for physical endurance. Additional systemic effects of training cannot be completely excluded. 31P magnetic resonance spectroscopy provides a noninvasive method for assessing this endowment.